Crossflow filtration is a type of membrane filtration that can be used when a fluid carries an amount of solid material that could plug a “dead end” filter. Crossflow filtration is different from dead end filtration. In dead end filtration, the feed is passed through a membrane or bed, trapping the retentate in the membrane or bed, and releasing the filtrate through the membrane or bed. Generally in dead end filtration, the only way for the feed fluid to exit the filter is through the membrane. In crossflow filtration, however, the feed is passed across the filter membrane (tangentially to the filter membrane) at some pressure, concentration, or other differential between the feed and the filtrate on the other side of the membrane. Material which is smaller than the membrane pore size passes through the membrane as filtrate. Some of the feed is trapped in or on the membrane as retentate, while the remaining feed flow passes across the filter on the feed side without passing through the membrane or becoming trapped in or on the membrane. The unfiltered feed flow exiting the filter is kept separate from the filtrate and may be recycled back through the filter. This mode of operation may be used for feeds with solids that cause a risk of “blinding.” Blinding is an accumulation of retentate on the membrane that fouls and/or reduces the effectiveness of a filter. With dead end filtration, solid material can quickly blind the filter surface, and feed flow can no longer pass through the membrane, rendering the filter ineffective. With crossflow filtration, the tangential motion of the bulk of the fluid across the membrane causes trapped particles on the filter surface to be removed by the tangential feed flow. This means that a crossflow filter can operate continuously with reduced blinding at a relatively high solids load compared to dead end filters.
Crossflow filter devices can take many shapes, including plates, hollow fibers, tubes and spirals. A spiral crossflow filtration device may include filter media wrapped around a permeate tube in a “jelly roll” type design. When looking at the end of a “spiral” filter, the end edges of the individual leaves of the filter element make a plane curve traced by a point circling about the center axis but at ever-greater distances from it. Each “leaf” is essentially hollow, like an “envelope” made out of filter media. In a spiral crossflow filter, the feed fluid flows parallel to the permeate tube. The feed flow enters one of the leaf “envelopes” across the filter media. The filtered fluid, or permeate, goes through the media and spirals between the media inside the leaf “envelope” and into the permeate tube. The permeate exits the filter through the permeate tube, and is kept separate from the remaining feed flow which exits the filter separately.
Crossflow membrane filtration technology has been used widely in industry globally. Cross flow filtration may be used, for example, in microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. There is still a need, however, for improved cross-flow filter devices.